There are various types of cylindrical batteries that are typified by a nickel-cadmium battery and a nickel metal hydride battery. Those cylindrical batteries are widely used in various applications, for example, as a power source for a cellular phone or a notebook computer, because they have high reliability and maintenance is easy. In recent years, development of cylindrical batteries suitable for large current discharge as a power source for an electric-assist bicycle, a lawnmower, and an electric vehicle has been demanded.
The cylindrical battery for a large current is formed by accommodating an electrode plate assembly in a metal battery case. The electrode plate assembly is formed by spirally winding a strip-like positive electrode plate and a strip-like negative electrode plate that are stacked with a separator interposed therebetween, which separates those electrode plates from each other. In a collecting structure suitable for inputting and outputting a large current, a positive electrode collector is welded at a plurality of locations to an end of the positive electrode plate projecting from an upper end face of the electrode plate assembly, and a negative electrode collector is welded at a plurality of locations to an end of the negative electrode plate projecting from a lower end face of the electrode plate assembly. The positive electrode collector includes a through hole formed at its center. The negative electrode collector 21 has a tongue-like connection piece 22 at its center, as shown in FIG. 12. The tongue-like connection piece 22 is formed by cutting. The electrode plate assembly to which those positive and negative electrode collectors are welded is accommodated in the metal case in such a manner that the negative electrode collector is arranged down. The positive electrode collector is connected to a sealing board via a connection lead. The negative electrode collector and the metal case are welded to each other by resistance welding. More specifically, the resistance welding is performed by making a welding current flow between a welding electrode bar inserted into the through hole of the positive electrode collector and a hollow cylindrical portion of the electrode plate assembly and a welding electrode arranged to be in contact with the bottom of the metal case, while the tongue-like connection piece 22 of the negative electrode collector and the inner bottom surface of the metal case are sandwiched and pressed between the welding electrode bar and the welding electrode.
Moreover, a structure is known, in which a contact concave portion projecting downward is formed at the center of the negative electrode collector and a protrusion for welding that protrudes inwardly is formed on the bottom of the metal case. Spot welding between the negative electrode collector and the metal case can be well performed by bringing the protrusion for welding and a lower surface of the contact concave portion into contact with each other (see Japanese Patent Laid-Open Publication No. 2000-106165, for example).
However, the aforementioned conventional structure has a problem that contact resistance between the metal case and the negative electrode collector is high because the metal case and the collector are connected to each other at one welding point. For example, when discharge is performed at a large current, e.g., 100 A, a voltage of the battery rapidly drops because of the high resistance at the portion where the metal case and the negative electrode collector are welded to each other. Moreover, when the tongue-like connection piece 22 is welded to the metal case, as shown in FIG. 12, resistance of the tongue-like connection piece 22 during the flow of current is large. Therefore, the tongue-like connection piece 22 may be melted and broken when a large current flows therethrough. When the protrusion for welding formed at the center of the bottom of the metal case is welded to the negative electrode collector, as disclosed in Japanese Patent Laid-Open Publication No. 2000-106165, an internal pressure of the battery increases because of overcharge or the like and causes expansion of the bottom of the metal case. Accordingly, deformation of the negative electrode collector occurs. This deformation may cause peeling of the negative electrode collector from the negative electrode plate. Thus, there is a possibility to increase in the battery resistance.
It is therefore an object of the present invention to provide a cylindrical battery and a method for manufacturing the cylindrical battery, which realizes low connection resistance between a metal case and a collector, suppresses voltage drop when large current discharge is performed, and suppresses deformation of the collector and ensure stable connection with the collector even when an internal pressure of the battery rises.